1. Field of the Invention
This invention concerns a device to remedy risks occurring during sluicing (lock-in) of long tools in a petroleum well, especially when using tools of a length extending through at least one of the well's wellhead valves during sluicing into the well.
2. Discussion of the Background Art
According to prior art of completing a petroleum well, a production tubing is set after cementing a casing in the formation. For a cased well there is no communication between the reservoir and the well at this stage. It is therefore necessary to perforate the well.
Wells are commonly perforated by means of firing directional explosive charges against the casing wall. The charges penetrate the casing wall, creating channels a distance into the reservoir. The reservoir fluid, for example oil, then may flow freely into the well.
To attain a best possible perforation, it is desirable to perforate the well while underbalanced, i.e. the pressure in the well is less than the reservoir pressure during the perforation operation. When perforating an underbalanced well, and immediately after the perforation, the well fluid will flush particles and slag formed during the perforation into the casing. This prevents the perforation and the formation adjacent to the casing from being blocked by said particles. For the same reason it is desirable that the entire reservoir zone be perforated simultaneously.
During operations in a pressurized well, and according to an ordinary requirement, at least two barriers arranged to prevent unwanted outflow from the well must exist at all times. Thus a safety valve, a so-called well safety valve (WSV), is positioned down in the well in addition to the wellhead valves already existing at the wellhead.
During lock-in of long tools in a well, typically a perforation gun, the tool may extend through the wellhead valves while the sluice opening still is open. During such operations, the well safety valve constitutes the only barrier between the well and the atmosphere. Should the perforation gun be dropped during sluicing, it may damage the well safety valve, thus further removing a barrier.
According to the prior art, so-called snubbing is used to lock-in long tools while the well safety valve is closed. Snubbing operations should be familiar to a specialist and are therefore not described in further detail. During snubbing operations, there is sometimes deviation from the requirement of at least two barriers between the reservoir and the atmosphere, the well safety valve providing the only barrier.
It is known to place devices in the well above the well safety valve for the purpose of protecting the well safety valve against falling objects. The devices may comprise closing mechanisms arranged to constitute an additional barrier. The applicant is not aware of devices of this type being used in active wells.
It is also known to install an additional well safety valve in the well above the primary well safety valve. This additional valve constitutes a backup valve to the primary well safety valve.
Both of the latter solutions are vulnerable and may be damaged by objects falling at high velocity.
The snubbing method is used to reduce the danger of dropping the tool string and thereby damaging the well safety valve. Should still the tool string be dropped, the tool most likely will damage the well safety valve. At worst, if the tool string comprises a perforation gun, it is conceivable that the perforation gun incorrectly is fired while being sluiced into the well. The wellhead valves then will be damaged. Thereafter pieces may fall down and damage the well safety valve. In the case of re-perforating an underbalanced well, an incident such as the one described above may cause loss of well control, render impossible to close the well.